The present invention relates to an apparatus for detecting an operation state of an MOS transistor, and more particularly relates to an apparatus for detecting whether or not an MOS transistor used as a constant current power supply in a CMOS circuit is operated in a saturated region (herein, being operated in a saturated region is referred to as a “saturated region operability”) and an apparatus for automatically adjusting an operation state of the CMOS circuit based on a result of the detection.
Conventionally, it has been known that in a CMOS circuit, a threshold voltage of an MOS transistor is changed due to process variations and temperature change and the like, and this change in threshold voltage influences transistor characteristics. With recent reduction in process size, a breakdown voltage of a gate oxide film and a power supply voltage have been reduced more and more, so that larger influence of the change in threshold voltage of an MOS transistor is given to transistor characteristics.
For example, in a constant current power supply that is an important element in an analog circuit, an MOS transistor constituting the constant current power supply has to be operated in a saturated region. However, due to reduction in operation range of a transistor caused by reduction in power supply voltage and change in threshold voltage and the like, it is difficult to ensure an operation in a saturated region. Therefore, means for detecting saturated region operability of an MOS transistor and, depending on cases, means for automatically adjusting an operation state of an MOS transistor to ensure saturated region operability are additionally required.
To cope with the above-described problems, in a known technique, a threshold voltage of an MOS transistor is measured using a measuring circuit such as an AD converter and a measurement result is fed back, so that an operation state of the MOS transistor is automatically adjusted (see, for example, page 7 and FIG. 1 of Japanese Laid-Open Publication No. 2002-76280). However, in this technique, an AD converter has to be provided, thus resulting in increase in circuit size and power consumption.